1. Field of the Invention
The present invention relates to a method and apparatus for capturing digital radiographic images. More particularly, the present invention relates to a method and associated apparatus for reading varying electrical charges representing a latent radiographic image stored in a unique microcapacitor matrix structure to obtain an electrical signal representing a radiogram.
2. Description of Related Art
Traditional radiography employs a silver halide photosensitive film in a light tight cassette enclosure to capture a latent radiographic image, which is subsequently rendered visible following chemical development and fixing. Because silver halide film is not very sensitive to X-ray radiation, and large exposures are required to obtain an image, most applications use a combination of an intensifying screen incorporated in the cassette with the silver halide film to achieve lower exposures. Intensifying screens typically comprise a phosphor layer coated over a supporting substrate. As a result of the impingement of X-ray radiation, the phosphor layers emit photons. The emitted photons, i.e., the light intensity output, are proportional to the X-ray energy level absorbed by the phosphor particles in the screen. The film sensitivity is adjusted to match the color output of the phosphor. The phosphor layer has a greater thickness than typical film emulsion and increased X-ray stopping efficiency. The overall response of the combined film screen system is thus greatly enhanced. In practice, X-ray films are often coated on both sides with photosensitive emulsion and two screens are used to expose the film from both sides, further increasing the efficiency of the system and reducing the exposure time required to obtain a radiogram.
Radiograms have also been produced by capturing a latent radiographic image using a photoconductive plate in a xeroradiographic process. In this instance, a photoconductive plate sensitive to X-ray radiation comprising at least a photoconductive layer coated over a conductive backing layer is first charged by passing under a charging station typically comprising a corona wire. Positive or negative charge is uniformly deposited over the plate surface. The plate is next exposed to X-ray radiation. Depending on the intensity of the incident radiation, electron hole pairs generated by the X-ray radiation are separated by a field incident to the charge laid over the surface and as a result move along the field to recombine with the surface charge. After X-ray exposure, a latent image in the form of electrical charges of varying magnitude remain on the plate surface, representing a latent electrostatic radiogram. This latent image may then be rendered visible by toning and preferably transferring onto a receiving surface for better viewing.
Xeroradiography, a specific application of electroradiography offers high resolution and, because the photoconductive layer may be made fairly thick comparative to the phosphor screens, results in good X-ray conversion efficiency. It is, however, limited by the same limitations found in xerography in general, i.e., dynamic range and the complexity of processing equipment. The photoconductive plates must be handled in the absence of actinic radiation until after toning. Furthermore, the image is not in a format that can readily provide a digital output, which is highly desirable for electronic image processing, storage and display.
More recent developments include the use of an electrostatic image capture element to capture a latent X-ray image, the element comprising a photoconductive layer over an insulating layer on a conductive support, the photoconductive layer also covered by a dielectric layer, and the dielectric layer overcoated with a transparent electrode. A biasing voltage is applied between the transparent electrode and the conductive support to charge the element which is a large parallel plate capacitor. While the bias voltage is applied, the element is exposed to image wise modulated X-ray radiation. Following exposure, the bias is removed and a latent image is preserved as a charge distribution trapped in the dielectric layer. The problem with this element structure is that the latent image represented by local charge variations is a very small signal charge that must be extracted in the presence of random noise in the total capacitive charge in the full plate. Signal to noise ratio is typically poor.
In an effort to improve the signal to noise ratio, the transparent electrode is laid over the dielectric layer as a plurality of parallel narrow strips. In this manner the overall plate capacity is reduced and the signal extracted per picture element has a better signal to noise ratio. Methods to readout the latent image include inter alia, scanning the length of the strip electrode with a laser beam while reading the charge flow across the electrode and the conductive plate. While this element is a vast improvement over the continuous electrode structure covering the full plate, the signal to noise ratio is still a problem because of the relatively high dark current leakage under the electrode strip. Thus, an X-ray capture system based on this element structure still suffers in image quality.